One object of the present invention is to provide an improved edge emitting semiconductor laser chip. A further object is to improve the strains and the wave-guiding in a semiconductor laser.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the edge emitting semiconductor laser chip comprises a carrier substrate.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the edge emitting semiconductor laser chip furthermore comprises an interlayer promoting adhesion between the carrier substrate and a component structure of the edge emitting semiconductor laser chip.
By way of example, the carrier substrate is mechanically fixed to a useful layer made of semiconductor material by means of the interlayer. The useful layer is preferably distinguished by a low dislocation density. That surface of the semiconductor layer which is remote from the carrier substrate then serves as growth area for epitaxially produced semiconductor layers—for example the component structure of the edge emitting semiconductor laser chip. It is furthermore possible for the carrier substrate to be bonded directly onto the epitaxially grown component structure of the edge emitting semiconductor laser chip by means of the interlayer.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the component structure comprises an active zone provided for generating radiation. That is to say that when the edge emitting semiconductor laser chip is energized, electromagnetic radiation is generated and/or amplified in said active zone, which radiation then leaves the edge emitting semiconductor laser chip as laser radiation.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the edge emitting semiconductor laser chip comprises a carrier substrate and an interlayer promoting adhesion between the carrier substrate and a component structure of the edge emitting semiconductor laser chip, wherein the component structure comprises an active zone provided for generating radiation.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer forms at least one part of a cladding layer of the edge emitting semiconductor laser chip. For this purpose, the interlayer has for example a significantly lower refractive index than the semiconductor material surrounding it. On account of the low refractive index of the interlayer, the optical wave generated in the semiconductor laser chip is shielded from the carrier substrate and also from layers lying between carrier substrate and interlayer, whereby so-called substrate modes—that is to say the penetration of the optical wave into the substrate—are effectively prevented. By virtue of the fact that the interlayer forms a part of the cladding layer of the edge emitting semiconductor laser chip, the thickness of an epitaxially grown cladding layer can be reduced.
In nitride-based laser diodes, for example, use is made of the materials InGaN for the active zone, GaN for the waveguide, the contact layer and the substrate, and AlGaN for the cladding layers. These materials have a different lattice constant in the unstrained state. This results in strains in the layer system. These strains in the layer system restrict the laser properties and possible growth parameters. The further the emission wavelength is shifted from approximately 405 nm—that is to say a little amount of indium in the active zone and a little amount of aluminum in the cladding layers—in the direction of blue or green (i.e. to wavelengths which are greater than 405 nm, the more indium is required in the active zone in order to set the emission wavelength. Since the refractive index contrast between GaN and AlGaN also decreases, however, as the wavelength increases, a higher aluminum proportion and/or a larger layer thickness is simultaneously required for comparably good wave-guiding. The lattice mismatch between AlGaN cladding layer and active zone—InGaN—is thus increasingly intensified with longer wavelength, whereby severe strains form in the material and limit the material quality. The same problem also arises analogously for short-wave lasers with an emission wavelength <405 nm. In particular for wavelengths <360 nm, it furthermore holds true that GaN contained in the laser structure absorbs greatly in this wavelength range. Therefore, defect-reduced AlGaN substrates are required for achieving high lifetimes. One possibility consists in growing the laser diodes on defect-reduced GaN, this being suitable with regard to the lattice constant primarily for low aluminum- and indium-containing layers. Thick cladding layers are particularly well suited to reducing the substrate mode. At long wavelengths, the required cladding thickness increases greatly and the filling factor in the active zone decreases.
The edge emitting semiconductor laser chip described here makes it possible, inter alia, for the substrate lattice constant to be set in a targeted manner. Furthermore, the edge emitting semiconductor laser chip enables for the first time, for lattice-matched epitaxy on low-defect substrates, inter alia, a reduction of the required cladding thickness and an increased filling factor in the active zone in conjunction with an improved strain balance of the active zone.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer forms a cladding layer of the edge emitting semiconductor laser chip. This embodiment of the edge emitting semiconductor laser chip completely dispenses with an epitaxially grown cladding layer on that side of the active zone which faces the interlayer. The interlayer here performs the task of the cladding layer. By virtue of the preferably great jump in refractive index between the interlayer and the waveguide of the semiconductor laser which faces the interlayer, the optical wave generated in the semiconductor laser is guided to a very great extent, whereby the filling factor in the active zone can advantageously be increased.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer is electrically insulating. In this case, the interlayer preferably contains a silicon nitride and/or a silicon oxide. In this case, the interlayer may for example contain one of the following materials or comprise one of the following materials: SiO2, Si3N4.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer provides an electrical contact between the carrier substrate and the active zone. That is to say that the interlayer is electrically conductive. On the p-side, for example, an electric current is impressed into the active zone of the edge emitting semiconductor laser chip via the interlayer. By way of example, the interlayer may in this case be formed from a transparent conductive oxide (TCO) such as ITO (indium tin oxide), for example, or contain this material.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer promotes adhesion between the carrier substrate and a useful layer, wherein at least one part of the component structure is deposited epitaxially onto that side of the useful layer which is remote from the interlayer. In this case, the carrier substrate may be for example a GaN carrier substrate that is cost-effective since it is high in defects. The carrier substrate is connected to the useful layer by means of the interlayer, which useful layer may be for example an epitaxially grown layer sequence that is low in defects and dislocations. At least one part of the component structure is deposited epitaxially onto this useful layer, which can then itself perform functional tasks in the edge emitting semiconductor laser chip.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the useful layer forms at least one part of a cladding layer or of a waveguide layer of the edge emitting semiconductor laser chip. This means that in the edge emitting semiconductor laser chip the useful layer performs the task of a cladding layer or of a waveguide layer. These layers or at least parts of these layers are then not produced together epitaxially with the remaining layers of the component structure, rather the component structure is applied, for example deposited epitaxially, onto these layers.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the interlayer is a bonding layer. That is to say that the interlayer serves for bonding the carrier substrate to the useful layer or directly to the component structure. By way of example for this purpose, the interlayer contains or consists of at least one of the following materials: SiO2, Si3N4, SiN, ITO, Al2O3, Ta2O5, HfO2, ZnO.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the refractive index of the material from which the interlayer is formed is less than the refractive index of the material from which the useful layer is formed. In this way, a jump in refractive index occurs between useful layer and interlayer, and, as described further above, can wholly or at least partly prevent the optical wave generated in the semiconductor laser chip from penetrating into the carrier substrate.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, the edge emitting semiconductor laser chip comprises two interlayers, wherein the active zone is arranged between the two interlayers. That is to say that an interlayer is arranged both at the p-siden-side and at the p-side of the edge emitting semiconductor laser chip, said interlayer forming a bonding layer for example. Preferably, the interlayers in this case are electrically conductive and serve for impressing a current into the active zone on the n-side and p-side.
In accordance with at least one embodiment of the edge emitting semiconductor laser chip, a material which is electrically insulating and/or has a smaller refractive index than the interlayer is partly arranged between the interlayer and the useful layer, or between the interlayer and the component structure. An index-guided edge emitting semiconductor laser chip, for example, is realized in this way.
In other words, the edge emitting semiconductor laser described here makes use of the following considerations, inter alia: an interlayer having a low refractive index, e.g. SiO2, Si3N4, ITO, is inserted into the laser structure. At the same time, the defect density of the epitaxial layers is kept low. For this purpose, by way of example, the laser structure is grown onto a useful layer with a low dislocation density, which is produced by a lateral cleave process. In this case, for bonding the useful layer onto the carrier, use is made of the interlayer having a significantly lower refractive index than GaN—for example SiO2, SiN, Si3N4, ITO—, which, in the edge emitting semiconductor laser chip, simultaneously acts as a cladding layer or at least as part thereof. On account of the low refractive index of the interlayer, in this case the optical wave is shielded from the underlying material—that is to say the carrier substrate for example, whereby a substrate mode is effectively prevented. In this case, the remaining cladding thickness directly above the interlayer can be chosen so as to set an optimum between optical and electrical properties. Thus, for better transverse conductivity, for example, a certain cladding thickness can remain above the interlayer. For the case of vertical current transport through the component for example with an ITO interlayer, it is possible to use a residual cladding layer thickness to prevent possible absorptions at the interlayer. However, the residual cladding layer can also be entirely omitted. In this case, the interlayer acts directly as a cladding layer, the optical wave being guided to a very great extent on account of the great jump in refractive index with respect to the waveguide, whereby the filling factor in the active zone can be increased. In this case, by virtue of the different possible cladding thicknesses, it is possible to set the strain in the active zone in a targeted manner, for example it is possible to set the strain without AlGaN cladding layer to relaxed GaN. Accordingly, a relaxed AlGaN substrate can be produced for UV lasers, and an indium-containing substrate can be produced for long-wave lasers.
The optimum between optical properties and good bondability can furthermore be set by using a plurality of layers, for example thin SiO2 for good bondability at the semiconductor interface, followed by a silicon nitride layer having a higher refractive index for matched wave-guiding.